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Stars, including our Sun, spend the majority of their lifetimes in the main sequence phase. During this stage, the star is in a state of hydrostatic equilibrium. This means gravity pulling inwards is balanced by the outward pressure from nuclear fusion reactions happening in the star's core. The primary fusion process is turning hydrogen into helium. This process releases energy that powers the star and makes it shine.
Stars can stay in the main sequence for millions or even billions of years, depending on their mass. More massive stars burn through their fuel much faster and leave the main sequence sooner. In contrast, smaller stars like red dwarfs can last for billions of years in this stage.
It's important to note that the main sequence is just the first stage in a star's life cycle. Eventually, the star will exhaust the hydrogen in its core, leading to changes in its structure and evolution.

After leaving the main sequence, a star enters the red giant phase. At this point, the core has exhausted its hydrogen fuel and can no longer sustain hydrogen fusion. As a result, the core contracts and heats up, while the outer layers expand dramatically.
In red giants, helium fusion begins in the core, creating heavier elements like carbon and oxygen. This expansion makes the star much larger in size and cooler in temperature, giving it a reddish appearance.
The red giant phase is a relatively short-lived stage compared to the main sequence. For our Sun, this phase will last only a few hundred million years. During this phase, the star experiences significant mass loss as its outer layers are blown away by strong stellar winds.

Once a star like our Sun has exhausted its helium fuel, it will shed its outer layers, creating what is known as a planetary nebula. The remnant core left behind is known as a white dwarf.
White dwarfs are incredibly dense objects where no further fusion occurs. Instead, they glow due to the residual heat from previous fusion reactions. Over time, white dwarfs cool down and become dimmer, eventually turning into black dwarfs, although the universe is not old enough for any black dwarfs to exist yet.
White dwarfs have masses similar to that of the Sun but are much smaller in size, comparable to Earth's radius. They represent the final evolutionary stage for stars that are not massive enough to end their lives in supernovae and become neutron stars or black holes.